1. Field of Invention
The invention relates to a correction method a, in which actuation characteristic curves and/or correction values are generated and are used during a pressure control process, to dimension the actuation current for one or more electrically actuated hydraulic valves which are operated in analogue fashion, in such a way that during the operation of an anti-lock brake system at first at least one or, in each case, one actuation characteristic curve is predefined and then the predefined actuation characteristic curve is corrected, in particular in the course of a learning process, characterized in that during a wheel pressure control process the current pressure model value for the controlled wheel is compared with a model locking-pressure level and/or analyzed.
2. Description of Related Art
In electronically controllable motor vehicle brakes having, inter alia, an anti-lock brake function (ABS), and also, if appropriate, vehicle movement dynamics control functions (ESP) etc., there is a continuously increasing need for improved quality of control, as a result of which, in addition to improved safety, the control comfort can also be improved. However, this requires usually complex and therefore costly hydraulic systems which have to be equipped with additional components (pressure sensors, control valves, switching etc.). It is therefore necessary to operate at least some of the valves in electronic brake systems in an analogue fashion.
EP-A-0 876 270 (P 8598) discloses the basic principle of a control cycle in an anti-lock brake system. According to the method described in said document, the gradient of the brake pressure build-up in the preceding brake pressure build-up phase is taken into account for the control of the brake pressure build-up during a control process in the current brake pressure build-up phase.
For example, WO 2004/101339 discloses that for the operation of the electrically actuated hydraulic valves which are operated in an analogue fashion it is important to set the magnetic field of the valve coil and therefore the tappet position as precisely as possible in order to produce a desired pressure gradient by means of the coil current through a current controller. The tappet position depends here in an extremely sensitive way on, inter alia, the current pressure conditions and valve-specific fabrication tolerances. If no directly measured pressure reference from the wheel brake cylinder is available, the brake system must know precisely the relationship between the opening current, differential pressure and volume flow for a valve which is operated in an analogue fashion. If this relationship is known, the control system can set the desired pressure gradient in a reproducible fashion with a defined coil current. In order to obtain the relationship between the valve current and the tappet position, complex measurements of the individual valve characteristics are generally necessary within the assembled brake system after the manufacture of the valves. However, this requires any manufactured brake control assembly to be connected to a test bench, which would have an undesirably large effect on the fabrication costs.
In order to solve this problem, WO 2004/101339 has proposed that, during the operation of the brake device, that is to say for example while the motor vehicle is travelling, an actuation characteristic curve should first be predefined and then during electronic brake control processes (for example during an ABS or ESP control process) said actuation characteristic curve, which is either stored at the factory or produced once by means of an additional calibration routine when the brake control device is activated, is corrected. For this “dynamic” correction, a learning process is carried out during which correction values are produced and used to correct the predefined actuation characteristic curve. During a pressure build-up phase, a current which corresponds to a certain volume flow is applied to the valve by the controller. This volume flow is determined by the controller as a function of the wheel pressure and the pressure/volume characteristic curve in such a way that the valve is operated at a stable point in the operating current characteristic diagram.
It has become apparent that the previously described correction method during operation does not always produce a valve characteristic curve with which volume flow can be set in an optimum way in terms of a high level of comfort and control quality.